The best solution seems to be to pay a little more and get the "enhanced" cameras like the DCS-942L. Those cameras do have rate control for both ftp and email. It's too bad these 93x cameras don't. This might be because the on-board memory for firmware is smaller. Probably due to this, these cameras don't have as many features as the enhanced models. But it's really too bad they didn't include a rate control. That's by far the biggest missing feature on these cameras.
I manage to get by with mine by using only email notification, using the 6 pictures per event, 1 second per picture setting. I use this to slow the camera down actually, because if I specify 1 email per event, the camera is capable of sending many more than 1 email per second. When I used this in combination with ftp, it seemed to slow the ftp down to the same one per second rate. Maybe when you enable both it tries to alternate between email and ftp? But I've stopped using ftp, and use only email now.
Maybe if you turn on both email, 6 pictures per event, and ftp also, you might slow the ftp rate to 1 per second. If you send the emails to a junk account, or maybe even an intentionally bad email server address, maybe this would slow the camera down. I have not tried sending to a bad email account or bad server, but I'd guess the camera might get slowed down trying. If it stops sending ftp while it tries to send an email, this might end up slowing the ftp rate down for you.